1. Field of the Invention
The invention relates to a circuit system for evaluating a sensor, wherein the circuit system has two complex impedances, wherein the complex impedances are each part of an oscillating circuit in which the complex impedances can be excited to oscillations and wherein at least one of the two complex impedances is a component of the sensor. In addition, the invention relates to a corresponding method.
2. Description of Related Art
Inductive sensors are widely used in practice. In this case, the reaction of a measured object is detected on a sensor coil and suitably converted into a measuring signal. Very frequently inductive sensors in the form of eddy current sensors are used that induce eddy currents in a conductive measured object located in the influence area of the sensor, which reacts to the sensor coil. Other sensors detect the change in inductance of the sensor coil caused by ferromagnetic materials. Inductive sensors are frequently used as position or proximity sensors.
During the evaluation of these sensors, the sensor coils are generally expanded to form an oscillating circuit and excited to oscillations. A measured object to be detected leads to a change in inductance of the sensor coil, whereby the frequency of the oscillation changes. By detection of the frequency detuning, the approach or the presence of a measured object can be concluded and, depending on the design of the sensor, a position and/or position change of the measured object or the respective physical variable to be measured is determined. Frequently the evaluation occurs by means of a PLL (phase-locked loop) that detects the deviation between an excitation frequency and the oscillation frequency of the oscillating circuit.
What is disadvantageous in the method for evaluating an inductive sensor known from practice is that the circuits necessary for this generally have a relatively complicated structure. For one thing, this causes a susceptibility to errors and, for another, the evaluation circuits cannot practically be used in the low cost area. Because of this, some application areas, e.g. the motor vehicle, are ruled out. Especially with the use of PLL circuits, the high temperature sensitivity also has a noticeable negative effect. In addition, the accuracy of the measuring results depends on the stability of the voltage supply of the circuit used, so complicated stabilizing measures are necessary.
Another general type of frequently used sensors uses a capacitive effect. In this case, the reaction of a measured object to the capacitance of a sensor is detected. Here as well the evaluation generally occurs with the use of an oscillating circuit, the characteristic values of which (frequency, amplitude and/or phase) change due to the measured object. The changes are usually determined by phase or amplitude modulation or by frequency measurements.
What is disadvantageous with the known circuits is that they are dependent on the voltage supply. Therefore, complicated measures are necessary in order to adequately stabilize the supply voltage. This in turn leads to comparatively complicated and expensive circuits that cannot be used in the low cost area.